Magnetotelluric study to characterize sediment thickness across Kachchh and Cambay rift basins, western India

Abstract

The Kachchh and Cambay rift basins are two pericontinental rift basins at the western continental margin of India (WCMI), which evolved during different stages of the Mesozoic era. Magnetotelluric measurements were carried out at 68 stations along four east– west trending profiles across these basins with an aim to infer basement configuration and sediment thickness. The data were analysed for galvanic distortions and decomposed into transverse electric (TE) and transverse magnetic (TM)-modes by rotating the impedance tensor into corresponding geoelectric strike directions of the four profiles. The decomposed data responses were then inverted using a nonlinear conjugate gradient algorithm. The top conductive layers (~2500–7500 S) across the Kachchh and Cambay rift basins indicate the presence of Cenozoic sediments and Deccan traps, which corroborates the results of earlier geophysical studies across these basins. The sediment thickness is low across Diyodar and Tharad ridges compared to the Sanchore, Patan and Mehsana sub-basins. A high conductive zone near Mehsana may support the evidence for the presence of Mesozoic sediments beneath traps as inferred from a deep seismic sounding (DSS) study. Even though the Oil and Natural Gas Corporation Limited (ONGC) drilled wells and DSS study reported the presence of Mesozoic sediments beneath the traps near Tharad ridge, their presence here is not clear from this study. Igneous intrusives and a Precambrian Aravalli–Delhi fold belt are delineated on either side of the Cambay rift basin. The electrical resistivity variations across these basins lead to the inference that the subsurface structure is highly heterogeneous in nature due to faults within the rift basins.